Formations in the earth are characterized by stress conditions which vary with depth and whose principle directions are generally vertical and horizontal. In the horizontal plane at any point, the horizontal stress reaches a maximum in one direction and a minimum at right angles to the maximum stress direction. Information concerning these maximum and minimum horizontal principle stress conditions is of substantial value in a variety of disciplines such as underground transportation systems, foundations of major structures, cavities for storage of liquids, gases or solids, and in prediction of earthquakes. Further, this information is essential in petroleum exploration and production, e.g. while drilling a well or borehole the information is useful for blowout prevention, in a completed well it is useful for evaluation of hydraulic fracture treatment, and also in determining many critically important aspects of reservoir behavior, such as bulk and pore volume compressibility, permeability, direction of fluid flow, and reservoir compaction/surface subsidence.
Currently, the technique of hydrofracturing is often used to measure the least principle stress in the plane normal to the borehole axis, i.e., the normal plane. In hydrofracturing, the least principle stress in a normal plane is measured with a borehole injection test where the fractures are caused by applying hydraulic pressure on the formation of the borehole. While these injection tests are an accurate means of determining in situ stresses and can be carried out a great depth, they are expensive and time consuming in that they require interruption of drilling to set borehole packers. Further, these tests are difficult to interpret.
In injection tests small volumes of fluid are pumped into small sections of the borehole, which are isolated by inflatable packers, with just enough pressure to create a small fracture. After each fracture of the formation, the pressure decline is measured as fluid leaks off. As long as a fracture is open, this pressure fall off should represent linear flow, and a plot of pressure fall off versus the square root of time should be a straight line. Once the fracture closes, the pressure fall off is no longer linear and the slope of the pressure fall off versus time plot will change. The point where this slope change occurs is interpreted to be the in-situ closure stress, which equals the minimum horizontal stress at that depth.
The use of inflatable packers to isolate a test interval in a borehole is not only time consuming but can present another problem as these packers may cause unwanted fracturing of the formation. This unwanted fracturing would mean that the results of the fractured tests are incorrect.
U.S. Pat. No. 5,511,615 issued on Apr. 30, 1996 to Douglas W. Rhett describes a method and apparatus for in-situ borehole stress determination. This method relies on a down hole jack to independently initiate three spaced apart fractures in a subterranean formation, measuring the breakdown pressure required to initiate each fracture, and finally using the measured breakdown pressures in two-dimensional axial transformation equations to compute the maximum and minimum principal stresses that are active in the plane normal to the borehole axis. The description of U.S. Pat. No. 5,511,615, showing the method for computing the maximum and minimum principal stress based on using measured fractured pressures in axial transformation equations is hereby incorporated by reference.
While the tool described and illustrated in U.S. Pat. No. 5,511,615 provided a substantial advance in the art of borehole stress determination, it has been found that conforming the face of the platens to a borehole wall is difficult because boreholes drilled through rock frequently have wall surfaces that are irregular and even non-cylindrical.
Accordingly it is the object of this invention to improve in-situ stress measurements in fracturing a selected location in a subterranean formation traversed by a borehole.
A more specific object of this invention to provide an expandable downhole tool having surfaces that better conform to an irregular wall of a borehole while applying pressure sufficient to initiate a fracture in the formation surrounding the borehole.
It is yet another object to allow accurate calculation of principle horizontal stress existing in a formation surrounding a borehole.